1. Field of the Invention
The present invention relates to a method for utilizing the heat of a low-temperature heat source such as a factory waste heat or a river water by the use of the chemical energy of a synthesis and decomposition reaction system of methyl formate, a method for preparing methyl formate and carbon monoxide, and a catalyst for a liquid phase reaction.
2. Description of the Related Art
Heretofore, for the recovery, transportation and utilization of heat energy, a technique of using water vapor or hot water has prevalently been employed. The application of this technique, however, has largely been restricted from the viewpoints of a heat loss and a facility cost, and hence, the recovery of the low-temperature waste heat has been limited.
That is to say, in various industrial facilities of a large energy consumption type such as power plants and iron works, saving energy has been advanced of late, and most of the waste energy has been recovered. However, the low-temperature waste heat of 200-300.degree. C. or less has been discarded owing to no means for properly utilizing such a waste heat in the self-facilities, so that a large cooling load has often been required.
In recent years, as a method for effectively recovering the low-temperature waste heat to utilize the waste heat for local air-conditioning, hot-water supply and the like, it has been investigated that the heat energy is converted into chemical energy to accomplish the heat recovery and heat utilization.
In this method, although the conversion between the heat energy and the chemical energy is necessary on a heat recovery side and a heat utilization side, transportation of heat energy for a long distance and storage of heat energy are possible, and the heat loss does not occur during the transportation and the storage. In addition, an efficiency of the heat transportation per unit volume in the above method is high. So the above method is also considered to be advantage from the viewpoint of the facility cost.
As an effective conversion system between the heat energy and the chemical energy, there has been suggested a method in which a methanol decomposition reaction of the formula (i) and a methanol synthesis reaction of the formula (i') are used: EQU CH.sub.3 OH = CO+2H.sub.2 (i) EQU CO+2H.sub.2 = CH.sub.3 OH (i')
In this method, since the methanol decomposition reaction of the formula (i) is an endothermic reaction, the waste heat can be recovered by the use of the reaction of the formula (i), and obtained CO and 2H.sub.2 can be transported. In a heat utilization area, the supply of the heat energy can be carried by the exothermic reaction of the formula (i'). Methanol produced by the formula (i') can be circulated to a heat recovery area and is reused therein.
In the conversion system using the formulae (i) and (i'), inexpensive and easily treatable methanol is used and the reaction can easily be carried out, and hence this conversion system is considered to be an effective energy conversion system. However, this kind of system has the following problems.
(1) The heat recovery is restricted by the lower limit temperature of the reaction of the formula (i), however, from the practical viewpoints of a reaction rate and the like, the lower limit temperature for the heat recovery is about 200.degree. C. So it is impossible to recover the heat in a low-temperature range of 200-100.degree. C. to nearly ordinary temperature which are considered to be the most suitable range for the waste heat recovery.
(2) In order to effectively do the heat recovery, it is necessary to decrease the reaction temperature of the formula (i) as low as possible, however, with the decrease in the reaction temperature, the equilibrium relation of the formula (i) becomes noticeably disadvantageous to a methanol decomposition side.
(3) In view of the heat utilization, it is advantageous to carry out the reaction of the formula (i') at a high temperature, however, with the rise in the reaction temperature, the equilibrium relation of the reaction of the formula (i') becomes noticeably disadvantageous to the synthetic reaction of methanol. Furthermore, in order to improve the equilibrium relation, the synthetic reaction of methanol is to be carried out under a high pressure, but the use of the high pressure deteriorates a heat utilization efficiency in view of a device cost, an operation cost and the like. Accordingly, the improvement of the reaction temperature and the pressure characteristics has been desired.
The present inventors have found that the recovery and the utilization of the waste heat can extremely advantageously be done by using, as the above conversion system of the heat energy into the chemical energy, a system of using methyl formate in place of methanol in the above process, i.e., a system including the decomposition of methyl formate into methanol and carbon monoxide and the synthesis of methyl formate from methanol and carbon monoxide represented by the following formulae, and the present inventors have already submitted a patent application (Japanese Patent Application Laid-open No. 42779/1997): EQU HCOOCH.sub.3 .fwdarw.CH.sub.3 OH+CO (I) (Endothermic reaction) EQU CH.sub.3 OH+CO.fwdarw.HCOOCH.sub.3 (I') (Exothermic reaction)
The method for obtaining carbon monoxide by decomposing methyl formate in accordance with the above formula (I) is noticed as a technique of easily obtaining carbon monoxide at a low temperature without using expensive facilities, because methyl formate can be obtained at a low cost from methanol as a main material in a Cl chemical industry and can be transported in the state of a liquid.
As the methods for obtaining carbon monoxide by decomposing methyl formate, there are known (1) a method which comprises thermally decomposing a gaseous phase at a temperature of 200 to 500.degree. C. by the use of a solid catalyst on which an oxide of an alkaline earth metal is supported (U.S. Pat. No. 3,812,210), (2) a method which comprises thermally decomposing a gaseous phase at a temperature of 200 to 550.degree. C. by the use of active carbon as a catalyst (Japanese Patent Application Laid-open No. 36609/1977), and (3) a method which comprises thermally decomposing a liquid phase of methyl formate coexisting with methanol at a temperature of 35 to 200.degree. C. under a pressure of 17.2 MPa or less by the use of sodium methylate as a catalyst (U.S. Pat. No. 3,716,619).
In the methods (1) and (2), the thermal decomposition is carried out at a temperature of 250.degree. C. or more in the gaseous phase, and latent heat for vaporization is required. Therefore, these methods are disadvantageous from the viewpoint of heat energy, and a large amount of impurities is secondarily produced, so that the selectivity of carbon monoxide is low. The above method (3) is excellent in that the reaction conditions of the liquid phase are mild, but it has drawbacks caused by a fact that sodium methylate is strongly basic. That is to say, impurities such as sodium hydroxide and sodium formate are secondarily produced owing to water during the reaction, which causes the catalyst loss, and some insoluble salts also precipitate in the reaction system, which brings about an operation trouble of an apparatus. In addition, during the use of the catalyst, if sodium methylate comes in contact with carbon dioxide or water, the catalyst is deactivated, and its regeneration is not easy. Moreover, since sodium methylate has a very strong irritation to a skin, much attention is required for its handling.
Methyl formate which is prepared by the formula (I') can effectively be utilized as a synthesis material of formic acid, formamide, various carboxylic esters and organic compounds, a solvent or the like, and hence, it is an industrially important organic chemical material.
As the preparation methods of methyl formate, there are known an esterification of formic acid with methanol, a dimerization of formaldehyde, an oxidation of methanol, a direct synthesis from hydrogen and carbon monoxide, a carbonylation of methanol with carbon monoxide, a dehydrogenation of methanol and the like. Among them, the methods which have been put to practical use on an industrial scale include the dehydrogenation method of methanol, and the carbonylation method of methanol which has been used for long years.
The dehydrogenation method of methanol is a novel method which has been put to practical use in recent years, and the development of catalysts having a high selectivity (e.g., Japanese Patent Application Laid-open Nos. 151047/1991 and 163444/1983) permits its practical utilization ["Chemistry and Industry", Vol. 18, p. 1134-1136 (1988)].
The carbonylation method of methanol by the formula (I') has been used for long years in Europe and America, and it is known as the main preparation method of methyl formate even at present. As catalysts to be used, there are known strongly basic catalysts such as metal alkoxides and DBU (diazabicycloundecene) as well as transition metal carbonyl compounds.
As the metal alkoxide catalyst for the carbonylation method, an alkoxide of an alkali metal is used, and a reaction is carried out at a temperature of 50 to 70.degree. C. under a pressure of 1 to 4 MPa [Journal of Molecular Catalysis, Vol. 18, p. 215-222 (1983)]. In the case that the metal alkoxide catalyst is used, water and carbon dioxide in methanol and carbon monoxide which are the raw materials function as catalyst poisons to deactivate and consume the catalyst. Therefore, it is necessary to decrease the amounts of water and carbon dioxide to the utmost, and hence a raw material purification system becomes an important step. Hydrogen present in carbon monoxide gas has not any influence on the reaction.
In the method using the metal alkoxide, the selectivity of methyl formate is high, and this method has actually been employed on an industrial scale. In order to advance the reaction more advantageously, many suggestions regarding reaction methods, structures of reactors, processes and the like have been given.
In the method using a strongly basic catalyst such as DBU, for example, the reaction is carried out at a temperature of 45 to 200.degree. C. under a pressure of 17.2 to 40.5 MPa in the presence of DBU in a methyl cellosolve solvent [Japanese Chemical Journal (4), p. 457-465 (1977)].
With regard to the metal carbonylation catalyst, a ruthenium hydride carbonium catalyst is considered to be effective for a formate synthesis by the carbonylation of an alcohol ["J. Mol. Catal.", Vol. 45, p. 235-246 (1988)].
In all of the above-mentioned methods, the reaction is carried out in a homogeneous catalyst system, but as a heterogeneous catalyst, a basic ion exchange resin is mentioned in U.S. Pat. No. 4,100,360.
In the above-mentioned carbonylation method of methanol, the metal alkoxide catalyst and the DBU catalyst which are strongly basic are used, and therefore this carbonylation method has the following drawbacks.
Firstly, the catalyst is liable to be affected by impurities in methanol and carbon monoxide gas which are the raw materials, and particularly, water and carbon dioxide react with the catalyst to change into insoluble substances, so that they cannot be separated any more, or salts of formic acid are secondarily produced. Consequently, in addition to the loss of the catalyst, even the operation itself of an apparatus might be impossible. In order to prevent this affection, it is necessary to completely purify methanol and carbon monoxide which are the raw materials, but for such a purification, a complicated process and a large amount of energy are required, which leads to an industrially large load. In addition, if the metal alkoxide comes in contact with CO.sub.2 or water during its handling and it deactivates, its regeneration is difficult. Moreover, since the metal alkoxide has a very strong irritation to a skin, much attention is required for the handling.
Furthermore, the basic ion exchange resin has a high initial activity and is excellent as the heterogeneous system catalyst, but its activity deteriorates with time. In addition, owing to a low heat resistance of the basic ion exchange resin, the quality change of the resin is unavoidable.